With a recent trend toward higher integration and higher density in semiconductor devices, circuit interconnects become finer and finer and the number of levels in multilayer interconnect is increasing. In the process of achieving the multilayer interconnect structure with finer interconnects, film coverage of step geometry (or step coverage) is lowered through thin film formation as the number of interconnect levels increases, because surface steps grow while following surface irregularities on a lower layer. Therefore, in order to fabricate the multilayer interconnect structure, it is necessary to improve the step coverage and planarize the surface in an appropriate process. Further, since finer optical lithography entails shallower depth of focus, it is necessary to planarize surfaces of semiconductor device so that irregularity steps formed thereon fall within a depth of focus in optical lithography.
Accordingly, in a manufacturing process of the semiconductor devices, a planarization technique of a surface of the semiconductor device is becoming more important. The most important technique in this planarization technique is chemical mechanical polishing. This chemical mechanical polishing (which will be hereinafter called CMP) is a process of polishing a substrate, such as a wafer, by placing the substrate in sliding contact with a polishing pad while supplying a polishing liquid containing abrasive grains, such as silica (SiO2), onto the polishing pad.
A polishing apparatus for performing CMP includes a polishing table that supports a polishing pad having a polishing surface, and a substrate holder, which is referred to as a polishing head or a top ring, for holding a wafer. When the wafer is polished with such a polishing apparatus, the polishing table and the polishing head are moved relative to each other while supplying the polishing liquid (slurry) onto the polishing pad disposed on the polishing table, and the wafer is pressed against the polishing surface of the polishing pad at a predetermined pressure by the polishing head. The wafer is brought into sliding contact with the polishing surface in the presence of the polishing liquid, so that the surface of the wafer is polished to a flat and mirror finish.
In such polishing apparatus, if a relative pressing force applied between the wafer and the polishing surface of the polishing pad during polishing is not uniform over the entirety of the surface of the wafer, insufficient polishing or excessive polishing would occur depending on the pressing forces applied to respective portions of the wafer. Thus, in order to even the pressing force applied to the wafer, the polishing head has a pressure chamber formed by an elastic membrane (or a membrane) at a lower part thereof. This pressure chamber is supplied with a fluid, such as air, to press the wafer against the polishing surface of the polishing pad through the membrane under a fluid pressure, and to polish the wafer.
Since the polishing pad has elasticity, the pressing force applied to an peripheral edge of the wafer during polishing of the wafer, becomes non-uniform, and hence only the peripheral edge of the wafer may excessively be polished, which is referred to as “edge rounding”. In order to prevent such edge rounding, a retainer ring for holding the peripheral edge of the wafer is provided so as to press the polishing surface of the polishing pad located at the outer circumferential edge side of the wafer.
A substrate transfer device, which is called a pusher, is disposed near the polishing table. This pusher has a function to elevate the wafer, which has been transported by a transporter, such as a transfer robot, and transfer the wafer to the polishing head that has been moved to a position above the pusher. The pusher further has a function to transfer the wafer, which has been received from the polishing head, to the transporter, such as a transfer robot.
In the polishing apparatus having the above-described structure, the wafer that has been polished on the polishing pad is moved to a position above the pusher by the polishing head. A wafer cleaning operation, a wafer releasing operation, and a polishing-head cleaning operation are then performed above the pusher.
In the wafer cleaning operation, a cleaning fluid, such as pure water, is ejected onto a polished surface of the wafer held by the polishing head, to thereby clean the polished surface of the wafer. After the wafer cleaning operation, the wafer releasing operation for releasing the wafer from the polishing head is performed. The wafer that has been released from the polishing head is received by the pusher and is then transported to a next process (e.g., cleaning of the wafer) by the transporter. After the wafer is released, the polishing-head cleaning operation is performed, in which a cleaning fluid, such as pure water, is ejected onto an outer surface of the polishing head to clean the polishing head, and further a cleaning fluid is ejected onto the membrane of the polishing head to clean a wafer holding surface of the membrane. According to this operation, the entirety of the polishing head including the membrane is cleaned.
Releasing of the wafer in the wafer releasing operation is performed by supplying a fluid into the pressure chamber to deform the wafer holding surface of the membrane. However, if the membrane is not deformed largely, the wafer may not be released from the membrane. Thus, in order to ensure releasing of the wafer from the polishing head, the pusher is provided with a release nozzle. This release nozzle is a device for delivering a jet of fluid (or releasing shower) into a gap between the wafer and the membrane to thereby assist the wafer release.
In the above-described wafer cleaning operation and polishing-head cleaning operation, the cleaning fluid is ejected onto the polishing head when the polishing head is located above the pusher. Therefore, the cleaning fluid, which has been brought into contact with the polishing head and the polished surface of the wafer, flows down onto the pusher. This cleaning fluid contains contaminants, such as abrasive grains and polishing debris which are attached to the polishing head and the wafer. Therefore, upon touching the release nozzle, the cleaning fluid may contaminate the release nozzle. In particular, the cleaning fluid containing the abrasive grains and the polishing debris may be sucked into the release nozzle from an opening thereof due to a capillary action, thus contaminating an interior of the release nozzle.
If the abrasive grains and the polishing debris are attached to the interior and a surface of the release nozzle, these abrasive grains and polishing debris may be attached to a next wafer together with the releasing shower, thus causing a contamination of the next wafer.
The releasing shower can assist the wafer release and can increase a throughput of polishing operation in the polishing apparatus. On the other hand, the releasing shower may hinder the wafer from being released if the releasing shower impinges on the surface (i.e., the polished surface) of the wafer, because the releasing shower is widened at a moment the releasing shower is expelled from a jet orifice of the release nozzle, thus pressing the wafer against the membrane.
In such a case, it has been a conventional solution to increase the pressure of fluid supplied into the pressure chamber of the membrane so as to inflate the membrane largely. When the membrane is largely inflated, the gap formed between the wafer and the membrane becomes larger, so that the releasing shower is less likely to impinge on the surface (the polished surface) of the wafer.
However, when the membrane is largely inflated while the wafer and the membrane are in an intimate contact, a large stress is generated in the wafer, causing a rupture of finer interconnects formed on the wafer or causing breakage of the wafer. Therefore, there is a demand for a technique which can properly eject the releasing shower into the gap between the wafer and the membrane even if the gap is small.
The releasing shower is widened while sucking surrounding particles. As a result, the releasing shower containing such particles is brought into contact with a front surface and a rear surface of the wafer, possibly causing the contamination of the wafer.